The role of miRNAs and piRNAs in planarian regeneration

Planarians are bilaterally symmetric metazoans that have the incredible ability to completely regenerate their entire body from a piece as small as 1/279th of a whole animal. This amazing regenerative capacity requires a population of stem cells called neoblasts. Several genes have been identified that are essential for regeneration in the planarian Schmidtea mediterranea. Conservation of most of these genes across the animal kingdom suggests that planarians are an excellent model system to study stem cell biology.^ MicroRNAs are small non-coding RNA molecules 21-24 nt in length that function by binding to the 3\u27UTRs of mRNAs where they prevent translation of the encoded protein or promote mRNA degradation. In other species, miRNAs have been shown to be involved in processes as diverse as fat metabolism, hematopoiesis, apoptosis, and the cell cycle.^ To begin studying the role of miRNAs in regeneration and stem cell function in planarians, we set out to identify miRNAs. During our experiments we sequenced over 2,400,000 small RNAs, which allowed us to identify 104 miRNA encoding loci in the S. Mediterranea genome. Our analysis of sequences from intact, regenerating, and irradiated planarians, has revealed dynamic changes in miRNA expression patterns. In particular, we found that members of the let-7 family of miRNAs appear to be expressed in neoblasts and therefore might be important regulators of stem cell function in planarians. Our sequencing efforts have also identified a second class of small RNAs in planarians called piRNAs. Intriguingly, in contrast to what is observed in other metazoans, in planarians, piRNA expression is not restricted to the germline, but rather appears to be specific to neoblasts. Therefore, piRNAs appear to be regulators of stem cell maintenance and/or self-renewal in planarians. ^ To further study the function of miRNAs in regeneration, we have cloned cDNAs encoding the miRNA synthesis machinery. Depletion of these proteins by RNA interference has revealed that a subset of these proteins are required for regeneration, homeostasis and proper miRNA synthesis. These experiments lay the groundwork for functional studies aimed at clarifying the role of miRNAs and piRNAs in regeneration, cell lineage decisions, and basic stem cell biology.